The major hypothesis of this work is that lead exposure is related to delinquent behavior. Our study of male primary school students not identified as delinquent has shown a relationship between bone lead concentrations, measured by X-ray fluorescence, aggression and delinquency. There was concordance in reports from three separate groups of informants: Parents' Child Behavior Checklist (CBCL), Teachers' CBCL, and subjects Self Report of Delinquency (SRD). This followed a developmental course. At age 7, parents and subjects found no association, and teachers reported a borderline association. At age 11 all three groups found a significant association after covariate adjustment. We shall follow these subjects into their 15th year of life, when the rate of delinquency increases substantially. We will measure the relative rate of increase in relation to lead exposure. We shall also extend our studies accomplished with nondelinquent to the most seriously delinquent youths in Allegheny County, PA. The X-ray fluorescence instrument will be placed in the Allegheny Academy, where 230 seriously delinquent, frequently violent adolescents receive strict supervision and rehabilitative services. Bone lead levels will be obtained. Measures of cognition, attention and language will be administered. This is a case control study. Controls will be students matched on gender and chosen by lot from the same public school class as the delinquents. Delinquents and controls will be evaluated by Mantel-Haenzel Procedures for stratified data, and by logistic regression, entering appropriate controls into the model. These data could have implications for understanding one of the causal factors of delinquent and aggressive behavior, and planning effective preventive strategies.